The present invention relates to a method for operating a internal combustion engine, in which a compression ratio is set to a setpoint compression ratio by means of an adjustment device, wherein in a normal operating mode of the internal combustion engine the setpoint compression ratio is determined as a function of an operating variable of the internal combustion engine. The invention further relates to a internal combustion engine.
The internal combustion engine serves for example to drive a motor vehicle. The internal combustion engine has a variable compression ratio, wherein the adjustment device is used to adjust the predetermined setpoint compression ratio at the internal combustion engine. When the setpoint compression ratio changes, the actual compression ratio of the internal combustion engine is adjusted to the setpoint compression ratio by means of the adjustment device. Hereby, however, the adjustment device only achieves a limited actuating speed. The setpoint compression ratio of the internal combustion engine is for example determined so that the fuel consumption is as low as possible. Thereby, however, the knock limit has to be taken into account. At a low load torque at the internal combustion engine, a higher setpoint compression ratio is selected than at a higher load torque. In particular, the setpoint compression ratio is adjusted to a maximum compression ratio of the internal combustion engine during a trailing throttle mode of the internal combustion engine, in which the internal combustion engine is carried along by an external torque, at idling speed and under partial load conditions up to a defined load torque.
When the load torque of the internal combustion engine is increased, for example due to a demand by a driver of the motor vehicle, the existing actual compression ratio has to be minimized as soon as possible in order to provide this load torque without exceeding the knock limit of the fuel. Due to the limited actuating speed, the ignition angle may for example be set during the time required for the actual compression ratio to reach the now lower setpoint compression ratio, so as to observe the knock limit, i.e., so as to avoid knocking of the internal combustion engine. For this purpose, the ignition angle is adjusted to “late”. On the other hand, however, when the load torque is decreased, the compression ratio is to be increased as fast as possible to reduce fuel consumption. However, due to the limited actuating speed the adjustment itself can only be realized with a higher fuel consumption.
When the operating point changes or the load torque changes slowly, this is unproblematic because the actuating speed of the adjustment device is sufficient. In the case of dynamic load changes on the other hand, i.e., a rapid change of the operating point or the load torque, the actual compression ratio lags far behind the setpoint compression ratio. In particular, the setpoint compression ratio is only determined from the operating variable of the internal combustion engine after reaching the new operating point and is subsequently set by the adjustment device thus leading to a significant time delay between the actual compression ratio and the optimal compression ratio.
From the state of the art the prior art patent document EP 1293 659 B1 is known. This document describes a control system for a internal combustion engine, which has a compression-ratio-control-device, an acceleration-parameter-attainment-device, an acceleration-determining-device as well as a control device. It is provided that the control device reduces a compression ratio reduction speed by which the motor compression ratio is reduced during motor acceleration, when the motor is in a slow acceleration mode as opposed to a compression ratio reduction speed with which the compression ratio is reduced during motor acceleration when the motor is in a fast acceleration mode. Furthermore, the control device is intended to delay a compression-ratio-reduction-start-time at which the reduction of the motor compression ratio is initiated during motor acceleration when the motor is in a slow acceleration mode, as opposed to a compression-ratio-reduction-start-time at which the reduction of the motor compression ratio is initiated during acceleration when the motor is in a fast acceleration mode.
Furthermore patent document DE 102 20 598 B3 describes a method to adjust the ignition angle to the compression ratio of a internal combustion engine, patent document DE 10 2004 031 288 A1 describes a internal combustion engine with variable compression ratio as well as a method for its operation and patent document DE 10 2011 017 181 A1 a method to operate an adjustment device to variably determine a compression ratio of a internal combustion engine.